The invention relates to a device for inserting and pushing a square bar into a press-piercing mill, the bar being disposed along the drilling and rolling axis of the mill.
A press-piercing mill is a machine adapted to convert a hot square bar into a round, axially bored bloom. The machine, which hereinafter will be denoted by the abbreviation PPM, mainly comprises a mill comprising round-grooved rolls, a chuck mounted along the rolling and drilling axis of the rolling mill and extending from the outlet side of the mill, a bit secured to that end of the chuck which faces the mill, and a substantially tunnel inlet guide coaxial with the rolling and drilling axis and adapted to guide a square bar to the mill.
In order to convert a square bar into a round, axially bored bloom in a PPM, the bar must be held exactly along the rolling axis and pushed between the grooves in the rolls and against the drill bit.
It is known that, in order to meet the rolling and drilling requirements in optimum manner, the axial thrust exerted on the bar must be considerable, whereas the bar must move between the rolls at a constant, relatively low speed (e.g. an average of 0.4 m/sec.).
To this end, in the prior art, a high-power hydraulic or mechanical actuator is used and is capable of slowly moving a carriage which is guided in a direction parallel to the PPM rolling axis, the carriage bearing a thrust rod having a longitudinal axis coinciding with the rolling axis.
The carriage is movable, relative to the PPM, from a remote position in which a space for receiving a square bar is left between the free end of the thrust rod and the inlet of the PPM inlet guide, and a position which is near the mill inlet but does not obstruct it.
The distance travelled by the bar, between the time when it is positioned between the thrust rod and the PPM inlet guide and the time in which it is completely converted into a bored round bloom, can be divided into the following two portions:
1. The bar-insertion portion.
This is the portion travelled between the mouth of the inlet guide and the place where the bar makes contact with the rolls, and
2. The thrust portion.
This is the remaining portion, along which the bar undergoes the desired conversion into a corresponding axially bored round bloom.
Portions 1, 2 are both travelled through at a reduced speed, i.e. the set speed of operation of the actuator. Furthermore, when a new square bar is positioned along the rolling axis upstream of the inlet guide, it is necessary to wait until the slide, moving at the reduced speed of the actuator, has returned to the position remote from the PPM so as to leave space for receicing a bar between the thrust rod and the PPM inlet guide.
The result is that the productivity of the PPM is greatly reduced by idle times due to the low speed with which, in the piror art, the carriage travels through the insertion portion and its entire return to the position remote from the PPM, the only exception being the thrust portion, along which the reduced speed is imposed by recognized structural operational requirements of the PPM.
This is the main technical disadvantage in the prior art insertion and thrust devices for press-piercing mills. The disadvantage is even more noticeable when the PPM is used to produce round, axially bored blooms for a tube rolling mill operating at the rate of four or more work-pieces per minute.